1. Field of the Invention
The present invention relates to particulates, particularly ultrafine particulates of titanium oxide obtained by a vapor phase process and a production process thereof. Moreover, the present invention relates to particulates, particularly ultrafine particulates of titanium oxide obtained from starting material of titanium tetrachloride, which particulates contain less aggregated particles and have excellent dispersibility. The present invention also relates to a production process of producing such particulates.
2. Description of the Related Art
Particulates, particularly ultrafine particulates of titanium oxide have very wide application areas in the industrial field and their diversified uses include an ultraviolet-shielding material, an additive to silicone rubber, a photocatalyst and the like. The xe2x80x9ctitanium oxidexe2x80x9d is referred to as xe2x80x9ctitanium dioxidexe2x80x9d in Japanese Industrial Standard (JIS) but the term xe2x80x9ctitanium oxidexe2x80x9d is used as a common name. Accordingly, this simple term xe2x80x9ctitanium oxidexe2x80x9d is hereinafter used in the present invention.
The importance of titanium oxide is increasing in the use for shielding an ultraviolet ray, for example, in the field of cosmetics, clothing and the like. As a shielding material, ultrafine particulates of titanium oxide are being used in many cases because of its high safety. For the shielding, two functions of absorbing and scattering the ultraviolet rays are necessary. The ultrafine particulates of titanium oxide have both of these two functions.
The titanium oxide has a property of absorbing ultraviolet rays at a wavelength of about 400 nm or less to excite electrons. When the electrons and the holes generated reach the surface of particulates, they combine with oxygen or water to generate various radical species. The radical species have an action of decomposing organic materials and therefore, in the case of using titanium oxide in cosmetics and the like, the ultrafine particulates of titanium oxide are generally surface-treated in advance. The fine particulates of titanium oxide are also used for making use of the photocatalytic reaction resulting from photoexcitation of titanium oxide. Furthermore, where titanium oxide is used for scattering ultraviolet rays, ultrafine particulates of titanium oxide having a primary particle size of about 80 nm are used. Although ultrafine particulates in general are not strictly defined with respect to the primary particle size, fine particles having a primary particle size of about 0.1 xcexcm or less are usually called ultrafine particles (particulates).
The titanium oxide is generally produced using a liquid phase process where titanium tetrachloride or titanyl sulfate as a starting material is hydrolyzed in a hydrophilic solvent or a vapor phase process where a volatile starting material such as titanium tetrachloride is vaporized and then reacted in the gas state with an oxidizing gas such as oxygen or steam at a high temperature. For example, JP-A-1-145307 discloses a method of producing ultrafine spherical particulates of metal oxide by setting the flow rate of either one of a volatile metal oxide and steam at 5 m/sec or more.
In general, the titanium oxide powder produced by the liquid phase process disadvantageously undergoes heavy aggregation. Accordingly, on use of titanium oxide in cosmetics and the like, the titanium oxide must be strongly cracked or pulverized and as a result, there arise problems such as mingling of abraded materials attributable to the pulverization treatment or the like, non-uniform distribution of the particle size, or bad touch feeling.
In the case of titanium oxide produced by the vapor phase process, the same problems as in the production by the liquid phase process will arise. That is, although ultrafine particulates of titanium oxide may be obtained by the conventional vapor phase process, only particulates of titanium oxide which have underwent grain growth can be obtained, so that for obtaining ultrafine particulates of titanium oxide, the titanium oxide must be strongly cracked or pulverized.
The present invention has been made to solve these problems and an object of the present invention is to provide particulates, particularly ultrafine particulates of titanium oxide which undergo considerably reduced aggregation and have highly excellent dispersibility.
Another object of the present invention is to provide a production process of producing such particulates of titanium oxide.
As a result of extensive investigations with view to solving the above-described problems, the present inventors have successfully found that in the vapor phase process, preheating each starting material gas can give rise to particulate, particularly ultrafine particulates of titanium having very excellent dispersibility.
More specifically, the process of producing titanium oxide of the present invention is characterized in that in the vapor phase process for producing titanium oxide by oxidizing titanium tetrachloride with an oxidizing gas at a high temperature, a titanium tetrachloride-containing gas and an oxidizing gas are reacted by supplying each gas into a reaction tube after preheating each gas at about 500xc2x0 C. or more to produce particulates, particularly ultrafine particulates of titanium oxide having a BET specific surface area of from about 3 m2/g to about 200 m2/g, preferably about 5 m2/g to about 200 m2/g, and more preferably about 10 m2/g to about 200 m2/g.
In the process, the preheated titanium tetrachloride-containing gas and the oxidizing gas may be supplied to a reaction tube each at a velocity of about 10 m/sec or more.
In the process, the titanium tetrachloride-containing gas and the oxidizing gas may be reacted by supplying these gases into a reaction tube and allowing them to stay there for about 3 seconds or less, preferably 1 second or less, and more preferably 0.5 second or less under a high temperature condition such that the temperature inside the reaction tube exceeds 600xc2x0 C.
The production process of particulates of titanium oxide of the present invention is characterized in that in the vapor phase process for producing titanium oxide by oxidizing titanium tetrachloride with an oxidizing gas at a high temperature, a titanium tetrachloride-containing gas and an oxidizing gas are each preheated at about 500xc2x0 C. or more, the preheated titanium tetrachloride-containing gas and the preheated oxidizing gas are supplied to a reaction tube at a velocity of about 10 m/sec or more, and these gases are reacted by allowing them to stay in the reaction tube at an average velocity of about 5 m/sec or more for about 3 seconds, preferably about 1 second or less, and more preferably 0.5 second or less under a high temperature condition such that the temperature inside the reaction tube exceeds about 600xc2x0 C.
In this production process, preferably, after each of the titanium tetrachloride-containing gas and the oxidizing gas is preheated at about 500xc2x0 C. or more, the preheated titanium tetrachloride-containing gas and the preheated oxidizing gas are supplied into the reaction tube to generate turbulence in the reaction tube.
In this process, the titanium tetrachloride-containing gas and the oxidizing gas are supplied into a reaction tube through a coaxial parallel flow nozzle and the inner tube of the coaxial parallel flow nozzle has an inside diameter of 50 mm or less.
In this process, the titanium tetrachloride-containing gas may contain from about 10 to 100% of titanium tetrachloride.
In this process, the titanium tetrachloride-containing gas and the oxidizing gas each may be preheated at a temperature of about 800xc2x0 C. or more.
The particulates, particularly ultrafine particulates of titanium oxide of the present invention has a BET specific surface area of from about 3 m2/g to about 200 m2/g, preferably from about 5 m2/g to about 200 m2/g, and more preferably from about 10 m2/g to about 200 m2/g, and a diameter corresponding to 90% of the particle size cumulative distribution on a weight basis as D90 diameter, of about 2.2 xcexcm or less.
The particulates, particularly ultrafine particulates of titanium oxide of the present invention has a BET specific surface area of from about from about 3 m2/g to about 200 m2/g, preferably from about 5 m2/g to about 200 m2/g, and more preferably from about 10 m2/g to about 200 m2/g, and a distribution constant n according to the following Rosin-Rammler formula of about 1.7 or more:
R=100exp(xe2x88x92bDn)
wherein D is a particle diameter, b is a constant, and n is a distribution constant.
The particulates of titanium oxide of the present invention can be produced by any one of the above-described processes.
The particulates of titanium oxide of the invention may be contained in various compositions.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments with reference to the attached drawings.